12 thoughts on “Proton soup – a turbulent, dynamically complicated structure”

1. Quanta Magazine > “Physicists Peer Inside a Fireball of Quantum Matter” by Charlie Wood (July 30, 2019) – Experimenters in Germany have glimpsed the kind of strange, non-atomic matter thought to fill the cores of merging neutron stars.

   Using an indirect computational method of examining the outcomes of collider experiments, the High Acceptance DiElectron Spectrometer (HADES) collaboration confirmed a “fireball” of particles corresponded to an exotic state of matter.

   … the HADES collaboration leveraged a different phenomenon: Almost as soon as the quark matter forms, it starts making short-lived composite particles called rho mesons, each composed of a quark and an antiquark. The rho mesons immediately transform into fleeting “virtual” photons, each of which splits into an electron and its antimatter twin, the positron. These particles carry information about the matter’s early moments all the way out to the HADES detector.

   The slope of the curve [a plot of the energies of electron-positron pairs] clocked the fireball’s temperature at hundreds of thousands of times that of the sun’s center, and the team calculated that its protons and neutrons reached a density analogous to the effect of cramming New York City into a sugar cube. At that density the protons and neutrons basically overlap, Galatyuk said. They don’t break up into free quarks, as happens in quark-gluon plasma (a phase of quark matter thought to have filled the universe in its first microseconds), but instead start to bunch up, acting more like clusters of six or nine quarks than triplets.

2. The Quanta Magazine article cited above (“Physicists Finally Nail the Proton’s Size, and Hope Dies” – September 11, 2019) noted that: “When an electron orbits the proton in the 2S state, it spends part of its time inside the proton.”

   Because electron orbitals can extend not just into an atom’s nucleus but inside its nucleons, I was reminded of this previously cited Gizmodo article about double electron capture:

   “Dark Matter Detector Makes Incredible Neutrino Observation” (4-25-2019).

   Scientists are pretty sure that the second most abundant particle in the Universe (after photons, particles of light) is the neutrino. … There are a ton of neutrino mysteries to solve. The new measurement, called “two-neutrino double electron capture,” is an important stepping stone to providing those answers.

   In the process, two protons in the atomic nucleus spontaneously and simultaneously absorb a pair of electrons orbiting the nucleus, releasing a pair of neutrinos. The experimental signature of the event is a barrage of x-rays and electrons resulting from other electrons orbiting the atom replacing the two absorbed by the nucleus. And when I say rare, I mean rare. The average amount of time it would take half of the xenon atoms in a sample to undergo this reaction is 1.8 × 1022 years, … roughly a trillion times the age of the Universe.

   Wiki > Double electron capture

   Double electron capture is a decay mode of an atomic nucleus. For a nuclide (A, Z) with number of nucleons A and atomic number Z, double electron capture is only possible if the mass of the nuclide of (A, Z−2) is lower.

   In this mode of decay, two of the orbital electrons are captured via the weak interaction by two protons in the nucleus, forming two neutrons. Two neutrinos are emitted in the process. Since the protons are changed to neutrons, the number of neutrons increases by two, the number of protons Z decreases by two, and the atomic mass number A remains unchanged. By changing the number of protons, double electron capture transforms the nuclide into a new element.

3. This article summarizes some research on the dynamics of proton soup – “the dance of quantum activity that includes virtual quarks … in the hubbub of virtual activity that surrounds all particles.” Feynman diagrams (from QED) are noted as a common mathematical tool to explore a particle’s “virtual entourage,” but may be questionable for QCD. Perhaps in part because, unlike photons, gluons can “trip over themselves” (in the gluon field).

   Quanta Magazine > “What Goes On in a Proton? Quark Math Still Conflicts With Experiments” by Charlie Wood (May 6, 2020): Two ways of approximating the ultra-complicated math that governs quark particles have recently come into conflict, leaving physicists unsure what their decades-old theory predicts.

   Feynman diagrams treat particles as if they interact by approaching each other from a distance, like billiard balls. But quarks don’t act like this. The Feynman diagram representing three quarks coming together from a distance and binding to one another to form a proton is a mere “cartoon,” according to Flip Tanedo, a particle physicist at the University of California, Riverside, because quarks are bound so strongly that they have no separate existence. The strength of their connection also means that the infinite series of terms corresponding to the Feynman diagrams grows in an unruly fashion, rather than fading away quickly enough to permit an easy approximation. Feynman diagrams are simply the wrong tool.

   “It’s tantalizing and frustrating,” said Mark Lancaster, a particle physicist based at the University of Manchester in the United Kingdom. “We know absolutely that quarks and gluons interact with each other, but we can’t calculate” the result.

   Some infer quark activity experimentally at particle colliders [colliding electrons and positrons], while others harness the world’s most powerful supercomputers [lattice QCD]. But these approximation techniques have recently come into conflict, leaving physicists unsure exactly what their theory predicts …

4. This Symmetry article contains one of the best lay descriptions of what happens when protons collide at nearly the speed of light. I’m still puzzled by how “throwing bosons back and forth” binds “particles” together – they carry a “stick together” message. I’d prefer an explanation using quantum field theory. As well as how colliding energetic gluons – field excitations – transform into other field excitations via “the wonders of quantum mechanics.” Fourier transform visualizations?

   Symmetry Magazine > “The large boson-boson collider” by Sarah Charley (04/30/2020).

   Normally scientists look at the particles produced during these [Large Hadron Collider] collisions to learn about the laws of nature. But scientists can also learn about subatomic matter by peering into the collisions themselves and asking: What exactly is doing the colliding?

   Protons are not solid spheres, but composite particles containing even tinier components called quarks and gluons. “As far as we know the quarks and gluons are point-like particles with no internal structure,” says Aram Apyan, a research associate at the US Department of Energy’s Fermi National Accelerator Laboratory.

   According to Apyan, two quarks cannot actually hit each other; they don’t have volume or surfaces. So what really happens when these point-like particles collide?

   “When we talk about two quarks colliding, what we really mean is that they are very close to each other spatially and exchanging particles,” says Richard Ruiz, a theorist at Université Catholique de Louvain in Belgium. “Namely, they exchange force-carrying bosons.”

   All elementary matter particles (like quarks and electrons) communicate with each other through bosons. For instance, quarks know to bind together by throwing bosons called gluons back and forth, which carry the message, “Stick together!”

   Almost every collision inside the LHC starts with an exchange of bosons (the only exceptions are when matter particles meet antimatter particles).

   The lion’s share of LHC collisions happen when two passing energetic gluons meet, fuse and then transform into all sorts of particles through the wonders of quantum mechanics.

   Gluons carry the strong force, which pulls quarks together into particles like protons and neutrons. Gluon-gluon collisions are so powerful that the protons they are a part of are ripped apart and the original quarks in those protons are consumed.

   In extremely rare instances, colliding quarks can also interact through a different force: the weak force, which is carried by the massive W and Z bosons. The weak force arbitrates all nuclear decay and fusion, such as when the protons in the center of the sun are squished and squeezed into helium nuclei.

   The weak force passes the message, “Time to change!” and inspires quarks to take on a new identity–for instance, change from a down quark to an up quark or vice versa.

   Although it may seem counterintuitive, the W and Z bosons that carry the weak force are extremely heavy–roughly 80 times more massive than the protons the LHC smashes together. For two minuscule quarks to produce two enormous W or Z bosons simultaneously, they need access to a big pot of excess energy.

   Even inside the LHC, weak force boson-boson collisions are extremely rare. This is because the range of the weak force extends to only about 0.1% of the diameter of a proton. (Compare this to the range of the strong force, which is equivalent to the proton’s diameter.)

   “This range is quite small,” Apyan says. “Two quarks have to be extremely close and radiate a W or Z boson simultaneously for there to be a chance of the bosons colliding.”

   Apyan studies collisions in which two colliding quarks simultaneously release a W or Z boson, which then scatter off one another before transforming into more stable particles. Unlike other processes, the W and Z boson collisions maintain their quarks, which then fly off into the detector as the proton falls apart. “This process has a nice signature,” Apyan says. “The remnants of the original quarks end up in our detector, and we see them as jets of particles very close to the beampipe.”

   The probability of this happening during an LHC collision is about one in a trillion. Luckily, the LHC generates about 600 million proton-proton collisions every second. At this rate, scientists are able to see this extremely rare event about once every other minute when the LHC is running.

5. So, stable matter depends on stable protons, a “soup” held together by quantum chromodynamics binding energy. The force is strong, eh. A complex dynamic quantum state which is the subject of active research.

   Then our everyday experience depends on the stability of nuclides, a composite of quantum states of nucleons, held together by nuclear binding energy. Yet another “Goldilocks” energy level for the dance of localized field excitations.

   But that medley can be jiggered. A proton (or neutron) can “drip” from a nucleus. (Find a visualization for that?)

   This Phys.org article discusses research that created atoms of exotic fluorine 25 (atomic number 9) from oxygen atoms (atomic number 8) by adding a proton. What caught my attention was that adding and then removing the extra proton did not restore the oxygen atom to its original quantum state: “the oxygen 24 at the core of the fluorine isotope mostly existed in excited states quite different from oxygen 24 itself.”

   Energy state(Oxygen 24) + Energy state(proton) = Energy state(Fluorine 25)

   Energy state(Fluorine 25 – proton) ≠ Energy state(Oxygen 24)

   Phys.org > “A single proton can make a world of difference” by Riken (May 28, 2020).

   Scientists from the RIKEN Nishina Center for Accelerator-Based Science and collaborators have shown that knocking out a single proton from a fluorine nucleus—transforming it into a neutron-rich isotope of oxygen—can have a major effect on the state of the nucleus. This work could help to explain a phenomenon known as the oxygen neutron dripline anomaly.

   The neutron dripline is a point where adding a single neutron to a nucleus will lead to it immediately drip a neutron, and this sets a limit on how neutron rich a nucleus can be. This is important for understanding neutron rich environments such as supernovae and neutron stars, since nuclei at the dripline will often undergo beta-decay, where a proton is converted into a neutron, driving it up the periodic table.

   Notes

   [1] Wiki: According to Byrne, stable nuclides are defined as those having a half-life greater than 10^18 years. A common example of an unstable nuclide is carbon-14 that decays by beta decay into nitrogen-14 with a half-life of about 5,730 years.

   The boundaries of the valley [valley of stability] correspond to the nuclear drip lines, where nuclides become so unstable they emit single protons or single neutrons.

   Wiki: “An arbitrary combination of protons and neutrons does not necessarily yield a stable nucleus.”

   Drip lines are defined for protons and neutrons at the extreme of the proton-to-neutron ratio; at p:n ratios at or beyond the drip lines, no bound nuclei can exist.

6. Hadrons galore!

   Protons are hadrons, composite particles made of quarks. But there are many other hadrons. So, why are protons and neutrons the only two that constitute our everyday world? Well, a stable world depends on stable particles, like the proton.

   This article contains a brief historical recap on looking to the skies for sources of radiation. And later the development of the quark model.

   • Symmetry Magazine > “Hundreds of hadrons” by Jerald Pinson (June 30, 2020) – Hadrons count among their number the familiar protons and neutrons that make up our atoms, but they are much more than that.

   Through decades of meticulous study, we now know that there are more than 100 different hadrons. By studying them, physicists have been able to paint a clearer picture of the four fundamental forces that explain our universe.

   Most hadrons are made up of either two or three quarks.

   Hadrons made up of three quarks—such as the proton and the neutron—are called baryons.

   Hadrons made up of two quarks are called mesons. These are bit more exotic; one of their two quarks is always an antimatter particle. Pions, for example, can either be positive, negative or neutral. Positive pions contain an up quark and an anti-down quark that are briefly pulled together in a delicate dance before decaying into a more stable form of matter.

   “The majority of a hadron’s mass actually comes from the energy of the gluons that bind quarks together,” says Cesar Luis Da Silva, a physicist at Los Alamos National Laboratory. “But exactly how the energy of gluons translates to the mass of hadrons is a question physicists are still trying to answer.”

   Hadrons made up of heavier quarks tend to be unstable due to their excess energy and thus exist only briefly before decaying into smaller particles. But the rate at which hadrons decay is governed by which force they interact with.

   “Neutral pions decay 300 million times faster than charged pions, even though they have the same mass,” says Da Silva. “That’s because neutral pions decay via the electromagnetic interactions, whereas charged pions decay through the weak force.”

   “Neutrons in the nucleus of atoms can live for quite a long time—up to billions of years—but as soon as they’re free of the nucleus, they decay in about 15 minutes,” he [Dmitri Denisov, deputy associate lab director, High-Energy Physics, Brookhaven National Laboratory] says.

7. Adding another decimal place to QED’s accuracy … 13 decimal places. Measurements on ordinary hydrogen vs. muonic hydrogen. Doppler-free two-photon frequency comb spectroscopy: pulsed vs. continuous wave laser and spectral range research.

   “The new result however suggests that the problem is of experimental rather than fundamental nature.”

   • Phys.org > “Unprecedented accuracy in quantum electrodynamics: Giant leap toward solving proton charge radius puzzle” by Katharina Jarrah, Max Planck Society (Nov 27, 2020)

   The new measurement is almost twice as accurate as all previous hydrogen measurements combined and moves science one step closer to solving the proton size puzzle. This high accuracy was achieved by the Nobel Prize-winning frequency comb technique, which debuted here for the first time to excite atoms in high-resolution [hydrogen] spectroscopy. The results are published today in Science.

   The electron in the hydrogen atom “senses” the size of the proton, which is reflected in minimal shifts in energy levels.

8. A further take on proton structure by Ethan Siegel.

   Forbes > “What’s Really Inside A Proton?” by Ethan Siegel Senior Contributor (Feb 15, 2021)

   (caption) The three valence quarks of a proton contribute to its spin, but so do the gluons, sea quarks and antiquarks, and orbital angular momentum as well. The electrostatic repulsion and the attractive strong nuclear force, in tandem, are what give the proton its size, and the properties of quark mixing are required to explain the suite of free and composite particles in our Universe. Individual protons, overall, behave as fermions, not as bosons.

   (caption) A proton isn’t just three quarks and gluons, but a sea of dense particles and antiparticles inside. The more precisely we look at a proton and the greater the energies that we perform deep inelastic scattering experiments at, the more substructure we find inside the proton itself. There appears to be no limit to the density of particles inside.

   (caption) When two protons collide, it isn’t just the quarks making them up that can collide, but the sea quarks, gluons, and beyond that, field interactions. All can provide insights into the spin of the individual components, and allow us to create potentially new particles if high enough energies and luminosities are reached.

9. If proton structure was effectively symmetric, then protons would lack charge. And for want of a charge, all matter be lost.

   • Quanta Magazine > “Decades-Long Quest Reveals Details of the Proton’s Inner Antimatter” by Natalie Wolchover (February 24, 2021) – Twenty years ago, physicists set out to investigate a mysterious asymmetry in the proton’s interior. Their results, published today, show how antimatter helps stabilize every atom’s core.

   Article has interesting visualization of the proton soup / sea of quarks and antiquarks. Fluid dynamics. (Reminds me of the boiling swirl of rice and water in a glass bowl cooking in my microwave. Energy feeding a mixture in a confined space.) Also more photos of the lab.

   In reality, the proton’s interior swirls with a fluctuating number of six kinds of quarks, their oppositely charged antimatter counterparts (antiquarks), and “gluon” particles that bind the others together, morph into them and readily multiply. Somehow, the roiling maelstrom winds up perfectly stable and superficially simple — mimicking, in certain respects, a trio of quarks. “How it all works out, that’s quite frankly something of a miracle,” said Donald Geesaman, a nuclear physicist at Argonne National Laboratory in Illinois.

   [The] SeaQuest [Fermilab experiment], has finally finished, and the researchers …measured the proton’s inner antimatter in more detail than ever before, finding that there are, on average, 1.4 down antiquarks for every up antiquark.

   Pion cloud model vs. statistical (gas) model [a distribution of speeds that depend on whether they possess integer or half-integer amounts of angular momentum] … QCD … self-dealing gluons … unsolvable equations … proton fluctuation between neutron-pion pair (toss a basketball to itself) … the antiquark’s momentum fraction … proton spin [bookkeeping or spin budget] …. SpinQuest …

   What’s at stake:

   SeaQuest’s hard data about the proton’s inner antimatter will be immediately useful, especially for physicists who smash protons together at nearly light speed in Europe’s Large Hadron Collider. When they know exactly what’s in the colliding objects, they can better piece through the collision debris looking for evidence of new particles or effects.

   Here’s another take on the Fermilab experiment.

   • Argonne National Lab > “Nature’s funhouse mirror: understanding asymmetry in the proton” by Savannah Mitchem (February 24, 2021) – Asymmetry [broken symmetry] in the proton confounds physicists, but a new discovery [an experiment at Fermilab] may bring back old theories to explain it.

   [A] ​”sea” of quarks and antiquarks popping in and out of existence is ever-present inside the proton.

   Curiously, at any given time, there are three more quarks than antiquarks: two more up quarks than anti-up quarks, and one more down quark than anti-down quarks. In other words, these mismatched quarks have no antimatter counterparts. This asymmetry is the reason protons are positively charged, allowing atoms — and therefore all matter — to exist.

   The experiment determined that there are always more anti-down quarks in the proton than anti-up quarks, no matter the quarks’ momentums.

   “You need experiment to lead the thinking and constrain theory, and here, we were looking for nature to give us insight into the proton’s dynamics,” said Geesaman. ​”It’s an interlacing cycle of experiment and theory that leads to impactful research.”

10. How do you squeeze or stretch a proton and detect such deformation?

    • Phys.org > “How stiff is the proton?” by US Department of Energy (October 3, 2022)

    (quote) This structure [the proton’s structure] deforms when exposed to external electric and magnetic (EM) fields, a phenomenon known as polarizability. The EM polarizabilities are a measure of the stiffness against the deformation induced by EM fields. By measuring the EM polarizabilities, scientists learn about the internal structure of the proton.

    Terms

    EM polarizability
    Effective Field Theories (EFTs)
    Quantum chromodynamics (QCD)
    Proton Compton scattering
    High Intensity Gamma Ray Source (HIGS)

    
    Credit: Pixabay/CC0 Public Domain

11. Here’s another case of thinking like a physicist – a scheme for describing a complicated quantum system in terms of a simpler one – when studying “low-energy” interactions. An application of perturbation theory. And years of analysis to validate those theoretical predictions.

    • Phys.org > “The direct measurement of a proton’s generalized polarizabilities in the strong quantum chromodynamics regime” by Ingrid Fadelli (November 4, 2022) – Direct tests of QCD are notoriously hard to come by.

    How exactly do the spin-dependent properties of the proton arise?

    Some aspects of the strong interactions in the quantum chromodynamics regime are still poorly understood, particularly when it comes to interactions at low energies and with low momentum transfer. One theory that makes predictions about nucleonic generalized polarizabilities (i.e., fundamental quantities describing the nucleon’s response to an external field in quantum chromodynamics), is chiral perturbation theory.

    Using the measurements they collected, Slifer and his colleagues [University of New Hampshire, University of Virginia, The College of William and Mary, and other institutes in the U.S. and China] were able to characterize the internal spin structure of individual protons …

    
    Credit: Jacek rybak, CC BY-SA 4.0, via Wikimedia Commons

12. 

    This graphic illustrates a proton moving at nearly the speed of light toward the viewer with its spin aligned along the horizontal direction (large arrow). The two views of concentric circles at the bottom show the spatial distributions of the momentum of up quarks (left) and down quarks (right) within this proton (white is high; violet is low). Credit: Brookhaven National Laboratory

    The interplay of theory and experiment in “imaging” a proton.

    How are quarks and the gluons distributed within the proton? Do up and down quarks contribute equally to a proton’s spin? Do gluons contribute to spin?

    Terms: quantum chromodynamics (QCD), lattice QCD, energy-momentum distributions, polarized protons.

    • Phys.org > “Calculations reveal high-resolution view of quarks inside protons” by Brookhaven National Laboratory (August 2, 2023) –

    A collaboration of nuclear theorists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, Argonne National Laboratory, Temple University, Adam Mickiewicz University of Poland, and the University of Bonn, Germany, has used supercomputers to predict the spatial distributions of charges, momentum, and other properties of “up” and “down” quarks within protons. The results, just published in Physical Review D, revealed key differences in the characteristics of the up and down quarks.

    … [accelerator] scatterings give scientists access to the Generalized Parton Distribution (GPD) of the proton – parton being the collective name for quarks and gluons. If you picture the proton as a bag filled with marbles representing quarks and gluons, the GPD provides a description of how the energy-momentum and other characteristics of these marbles are distributed within the bag … the likelihood of finding a marble with a specific energy-momentum at a particular position inside the bag.

    According to their calculations, the scientists concluded that up and down quarks can account for less than 70% of the proton’s total spin.

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